Hetero bipolar transistor

Abstract

In a hetero bipolar transistor according to the present invention, a SiGe spacer layer ( 40 ), a SiGe graded layer ( 41 ) having a portion functioning as a base layer and composed of a plurality of sublayers having different Ge composition ratios, and a Si cap layer ( 42 ) are formed on a collector layer ( 12 ) formed in a portion of a Si substrate ( 10 ). In the SiGe graded layer ( 41 ), when the thickness of each sublayer is larger, the number of sublayers is smaller, and difference in Ge composition ratio between adjacent sublayers is larger, measurement of the thickness of each sublayer becomes easier. However, in order to avoid degradation of a cut-off frequency characteristic of a device, the film thickness of each sublayer needs to be approximately 20 nm or less, and the number of sublayers is preferably 2 or more.

What is claimed is:  
     
         1 . A hetero bipolar transistor comprising: 
 a semiconductor substrate;    a first semiconductor layer formed on the semiconductor substrate and comprised of a crystal containing silicon and germanium;    a second semiconductor layer formed on the first semiconductor layer and comprised of a crystal containing silicon and germanium, at least a portion of the second semiconductor layer functioning as a base layer; and    a third semiconductor layer formed on the second semiconductor layer and comprised of a crystal containing silicon, at least a portion of the third semiconductor layer functioning as an emitter layer, wherein the second semiconductor layer includes regions each having a germanium composition ratio that varies stepwisely with a difference of 2.5% or more, in the vicinity of a boundary between the first semiconductor layer and the second semiconductor layer and a boundary between the second semiconductor layer and the third semiconductor layer.    
     
     
         2 . A hetero bipolar transistor comprising: 
 a semiconductor substrate;    a first semiconductor layer formed on the semiconductor substrate and comprised of a crystal containing silicon and germanium;    a second semiconductor layer formed on the first semiconductor layer and comprised of a crystal containing silicon and germanium, at least a portion of the second semiconductor layer functioning as a base layer; and    a third semiconductor layer formed on the second semiconductor layer and comprised of a crystal containing silicon, at least a portion of the third semiconductor layer functioning as an emitter layer, wherein the second semiconductor layer includes a region having a germanium composition ratio that varies stepwisely with a difference of 2.5% or more, in the vicinity of a boundary between the first semiconductor layer and the second semiconductor layer.    
     
     
         3 . A hetero bipolar transistor comprising: 
 a semiconductor substrate;    a first semiconductor layer formed on the semiconductor substrate and comprised of a crystal containing silicon and germanium;    a second semiconductor layer formed on the first semiconductor layer and comprised of a crystal containing silicon and germanium, at least a portion of the second semiconductor layer functioning as a base layer; and    a third semiconductor layer formed on the second semiconductor layer and comprised of a crystal containing silicon, at least a portion of the third semiconductor layer functioning as an emitter layer, wherein the second semiconductor layer includes a region having a germanium composition ratio that varies stepwisely with a difference of 2.5% or more, in the vicinity of a boundary between the second semiconductor layer and the third semiconductor layer.    
     
     
         4 . The hetero bipolar transistor according to  claim 1 , wherein the second semiconductor layer contains silicon, germanium, and carbon.  
     
     
         5 . The hetero bipolar transistor according to  claim 1 , wherein the second semiconductor layer is comprised of a plurality of sublayers having different germanium composition ratios, and the number of the sublayers is not less than 2 and not more than 6.  
     
     
         6 . The hetero bipolar transistor according to  claim 1 , further comprising a marker layer formed between the first semiconductor layer and the second semiconductor layer, wherein the marker layer has a germanium composition ratio higher or lower than a germanium composition ratio of the first semiconductor layer by 2.5% or more and is comprised of a crystal containing at least silicon and germanium.  
     
     
         7 . The hetero bipolar transistor according to  claim 1 , further comprising a marker layer formed between the second semiconductor layer and the third semiconductor layer, wherein the marker layer has a germanium composition ratio higher than a germanium composition ratio of the third semiconductor layer by 2.5% or more and is comprised of a crystal containing at least silicon and germanium.  
     
     
         8 . A hetero bipolar transistor comprising: 
 a semiconductor substrate;    a first semiconductor layer formed on the semiconductor substrate and comprised of a crystal containing silicon and germanium;    a second semiconductor layer formed on the first semiconductor layer and comprised of a crystal containing silicon and germanium, at least a portion of the second semiconductor layer functioning as a base layer; and    a third semiconductor layer formed on the second semiconductor layer and comprised of a crystal containing silicon, at least a portion of the third semiconductor layer functioning as an emitter layer,    wherein the second semiconductor layer includes regions each having a bandgap that varies stepwisely with a difference of 18 meV or more, in the vicinity of a boundary between the first semiconductor layer and the second semiconductor layer and a boundary between the second semiconductor layer and the third semiconductor layer.    
     
     
         9 . A method of measuring a thickness of a semiconductor layer in a hetero bipolar transistor including: 
 a semiconductor substrate;    a first semiconductor layer formed on the semiconductor substrate and comprised of a crystal containing silicon and germanium, the germanium having a constant concentration;    a second semiconductor layer formed on the first semiconductor layer and comprised of a crystal containing silicon and germanium, at least a portion of the second semiconductor layer functioning as a base layer; and    a third semiconductor layer formed on the second semiconductor layer and comprised of a crystal containing silicon, at least a portion of the third semiconductor layer functioning as an emitter layer,    the method comprising measuring a thickness of the second semiconductor layer,    wherein the second semiconductor layer is comprised of a plurality of sublayers,    a composition ratio of germanium contained in a first sublayer is different from a composition ratio of germanium contained in a second sublayer adjacent to the first sublayer, and    a first boundary between the first semiconductor layer and a sublayer of the second semiconductor layer which is adjacent to the first semiconductor layer, where the composition ratio of germanium varies discontinuously, and a second boundary between the third semiconductor layer and a sublayer of the second semiconductor layer which is adjacent to the third semiconductor layer, where composition ratio of germanium varies discontinuously, are detected by using a spectroscopic ellipsometer, to define a distance between the first boundary and the second boundary as the thickness of the second semiconductor layer.    
     
     
         10 . The method of measuring a thickness of a semiconductor layer in a hetero bipolar transistor according to  claim 9 , wherein each of the plurality of sublayers contains germanium of a composition ratio of 1.5 % or more, and a difference in composition ratio between the germanium contained in the first sublayer and a composition ratio of the germanium contained in the second sublayer is 1.5% or more.  
     
     
         11 . The method of measuring a thickness of a semiconductor layer in a hetero bipolar transistor according to  claim 9 , wherein each of the plurality of sublayers contains germanium of a composition ratio of 2.5 % or more, and a difference in composition ratio between the germanium contained in the first sublayer and the germanium contained in the second sublayer is 2.5% or more.